It is generally known in the art of manufacturing automotive glazings that to form a glass sheet templet into suitable shapes, as e.g, a backlite, the glass sheet templet is first heated to a temperature above its plastic set temperature, usually about 1200.degree. F., then shaped to a desired curvature by either gravity forming or press bending the hot glass, and thereafter tempered by directing streams of a tempering fluid, usually moist air, against the major surfaces thereof. It is well-known that ceramic materials are much stronger in compression than in tension. Therefore, "tempered" glass is typically used for vehicle glazings, architectural uses such as glass doors, and other high-strength-requirement applications.
During tempering, residual compressive stresses are intentionally induced in the shaped glass sheet. The major surface regions of the glass sheet contract because of the drop in temperature as a result of convective heat transfer to the cooling air. Thus, the major surface regions of the glass sheet become rigid, while the central portion of the glass sheet remains hot and can adjust its dimensions to the surface region contractions. When the central region of the glass sheet cools and contracts slightly at a later time, compressive stresses are produced in the outer major surface regions of the glass sheet.
A constant cooling rate applied to both major surfaces of the shaped glass sheet, resulting from an identical flow of constant-temperature cooling air to both major surfaces, theoretically would produce a parabolic stress distribution when measured normal to the major surfaces of the glass sheet.
Tempered glass is particularly useful for high-strength applications because the exposed surfaces of the tempered glass sheet are under residual compressive stress. Glass failure usually occurs from an applied tensile (rather than compressive) stress. Since failure in a tempered glass sheet almost always is initiated at one of its major surfaces, e.g., by striking the tempered glass sheet, any applied stress must first overcome the residual compression near the surface of the tempered glass sheet before that region is brought into tension such that failure may occur.
During tempering, it is known to support the shaped glass sheet on a support ring, comprising a rigid structure conforming generally in outline and elevation to the underside peripheral marginal surface of the shaped glass sheet. During the tempering operation, the blasts of tempering fluid rapidly cool the major surfaces of the formed glass sheet in all areas, except those areas near points of contact between the tempering support ring and the underside peripheral marginal surface of the glass sheet. In those areas, cooling is retarded due to the restricted flow of tempering fluid caused by interference with the tempering support ring. As discussed above, after tempering the outermost layers of the tempered glass sheet are in compression while the central layer between the compression layers is in tension, however, those layers of the ultimately produced tempered glass sheet may be stressed in compression/tension in other than optimal numerical amounts and the thickness of the compression layers may be less than optimal for proper tempering strength.
Moreover, other variables in the tempering process can result in poor quality and nonuniform tempering, wherein the configuration of the actual stress (distribution numerical value and thickness of the tension and compression layer) measured across the thickness of the glass sheet at any point along the surface of the tempered glass sheet varies markedly from an idealized parabola. Such a stress imbalance and less than desirable thickness of the compression layers may lead to spontaneous breakage of the tempered glass sheet. Clearly, it is important to be able to modify the tempering conditions as necessary to produce an optimally tempered glass sheet. In order to be able to do this, it is necessary to be able to accurately and conveniently measure the value of the tensile stress in the glass and the thickness of the various layers. It would be desirable to devise such a method for determining these parameters. One process for determining the temper quality has been described in U.S. Pat. No. 5,254,149 to Hashemi et al. which is commonly assigned with the present invention. It teaches a process which involves repeatedly scoring a major surface of a tempered glass sheet with a laser beam until the tempered glass sheet shatters. The patent discloses determining temper quality by comparison between the scoring required to shatter that tempered glass sheet and that required to shatter another tempered glass sheet of desirable quality.
Unexpectedly, we have found that when glass is tempered optimal strength is induced when certain percentages of the thickness of the tempered glass sheet are respectively in compressive stress and in tensile stress. In addition, we have found that proper tempering and hence strength of the tempered glass product are related to an optimal magnitude of the tensile stress, so that there exists a correlation between the compression layer thickness (%) and tension layer stress in properly tempered glass. Prior to this invention, technology has not been suitably capable of determining the thickness of the compression layer (%) as a function of the values of tensile membrane stress in the tempered glass. This invention is capable of determining both, even in glass having a paint on one of its surfaces. Automotive glazing often include a painted portion on the outer areas of such glazings.